The introduction of the 16:9 wide aspect ratio image format into existing television standards with a 4:3 aspect ratio format (for example PAL, SECAM, NTSC) has resulted in the so-called "letter box" image display format. This format is well known and is discussed, for example, in "Improvement Options and Development Tendencies in PAL", G. Holoch, Lecture FKTG 17 January 1989 in Berlin, Germany; "Future Television Systems", F. Muller-Romer, Fernseh-und Kinotechnik, Vol. 43, No. 6/1989; and "New Paths of the Old PAL", Dr. A. Ziemer etal, Funkschau No. 18/1989. In the compatible letterbox format a 16:9 image is displayed by a 4:3 receiver in a central display area, with bar regions along upper and lower image edges containing other information which is typically suppressed so as to be invisible to a viewer.
Illustratively, at a studio (transmitter) a 16:9 interlace image with 625 total lines and 575 active image lines is generated. Every fourth imaqe line is frame-wise removed and transmitted as additional information with reduced amplitude in the bar regions above and below the active central image area. In such case the active image area in a 4:3 receiver comprises 431 lines, and the bar regions each contain 72 lines. With this format a 16:9 image can be displayed compatibly by a 4:3 receiver without any readjustment on the part of the viewer. The ratio of original number of lines to the reduced number of lines, and geometric distortion is compensated by removing lines. A 16:9 receiver processes the information contained in the bar regions and increases the number of the active lines, which had been reduced to 431, back to 575, thereby generating a 16:9 image in interlace form which occupies the 16:9 display area. However, process-dependent artifacts can be present, and the disadvantages of the interlace process (for example, line and edge flicker) are still present.